The major goals of this proposal are (1) to develop and validate laser speckle imaging (LSI) for repeated measurements of absolute blood velocity in microvessels and collateral arteries and (2) to provide, through the use of these LSI methods, insight into how shear stress regulates microvascular remodeling and arteriogenesis. LSI utilizes the speckle pattern generated by the interaction of coherent laser light with tissue. As red blood cells in the tissue move, the image exhibits a decrease in speckle contrast and variation, which can be related to blood flow. LSI has been used previously to measure relative flow changes; however, based on preliminary comparative microparticle image velocimetry (?PIV) studies, we believe that absolute blood flow measurements with LSI may be feasible and applicable to in vivo models of both microvascular remodeling and arteriogenesis. Our overall hypothesis is that LSI can be used to measure and track shear stress levels in individual microvessels and collateral arteries as these vessels structurally remodel through time. In Aim 1, we will optimize and utilize LSI to determine shear stress levels in individual microvessels in mouse dorsal skinfold window chamber as they remodel in response to a hemodynamic perturbation, which will be generated by micro-occluding an arcade arteriole or venule. Microvascular remodeling is essentially a long-term autoregulatory response that maintains tissue perfusion; however, dysregulated microvascular remodeling can facilitate and/or exacerbate many pathological conditions. Shear stress is a likely regulator of functional microvascular remodeling; however, the molecular transducers linking microvascular remodeling to shear stress are essentially unidentified. We contend this is primarily because no low-cost experimental approaches capable of linking absolute shear stress changes to microvascular remodeling in individual vessels over time currently exist. The results from Aim 1 will fill this critical knowledge gap and enable, for the first time, in vivo experimental investigtion into microvascular adaptation. In Aim 2, we will use an optimized LSI approach to determine how arteriogenesis and monocyte recruitment are regulated by alterations in shear stress magnitude and/or direction. In the context of the treatment of peripheral arterial disease (PAD), therapeutic arteriogenesis clinical trials have been unsuccessful. We contend this is due largely to our continued poor understanding of the endogenous response. Intriguing new studies from our lab show that collateral segments exposed to shear reversal after upstream arterial occlusion exhibit markedly accelerated arteriogenesis. Uncovering why shear reversal accelerates arteriogenesis could provide important clues for improved therapeutic approaches. The studies in Aim 2 will yield detailed shear-growth histories that can then be used to understand how endothelial cells sense and respond to these precise changes in shear stress magnitude and direction. In essence, this will provide the necessary foundation for mechanistic studies aimed at deciphering which signaling pathways link shear stress to accelerated arteriogenic remodeling.